III-V materials such as GaN are particularly useful in constructing lasers and LEDs in the blue and green wavelengths. To simplify the following discussion, a reference to GaN is to be understood as including the entire GaN material system of GaN, AlN, InN, AlGaN, InGaN, AlGaNP, AlGaNAs, InGaNP, InGaNAs, etc. unless the context makes it clear that only GaN is intended. These layers are typically deposited on a sapphire substrate. Consider a layer of GaN grown on a sapphire substrate. The growth temperature is typically in the range of 700 to 1200.degree. C. The thermal coefficient of expansion of GaN and AlN is significantly different than that of sapphire along certain planes. Hence, when the substrate and GaN layers are returned to room temperature a significant thermal stress is applied to the GaN layer. This stress leads to fractures and voids in the GaN layer. In addition, the wafer may be warped by the stress generated at the GaN/sapphire interface.
In addition to thermal stress, stress resulting from the mis-match of the lattice constants between the GaN and sapphire layers also causes problems. For example, the lattice constant for GaN differs from that of sapphire by approximately 13-16%. During the growth of the GaN layer, the substrate and the GaN layer tend to keep their original lattice parameters; hence, there is a mismatch between the lattice parameters between the two layers. This mismatch generates stress in the GaN layer which, in turn, leads to defects in the layer.
Broadly, it is the object of the present invention to provide an improved method for growing group III-V layers on a substrate having a substantially different coefficient of thermal expansion and/or lattice constant than the layer being grown.
It is a further object of the present invention to provide a method for growing GaN on sapphire in which the thermal stress generated by growing the layer can be relieved.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.